1. Field of the Invention
The present invention relates to the determination of the boundaries of underground reservoirs and more particularly to a method of determining the locations of the gas/water interfaces in an underground gas reservoir by measuring surface potentials by means of a system of measuring probes.
2. Prior Art
Well logging and seismic surveys have hitherto been the only two methods of measuring the expansion of gas in an underground aquifer storage reservoir, the latter method being the only conventional above-ground method. In such seismic surveys, the presence of gas is identified by changes in the reflection configurations and the reflection amplitudes of seismic waves. For the application of said seismic technique, it is necessary to make a reference survey prior to the presence of gas in such a reservoir. Through the use of an appropriate grid, gas expansion in the storage horizon for example during the development of an underground storage reservoir may be monitored from time to time. Such seismic surveys and the interpretation of the data of such surveys are complicated and costly. Conventional seismic surveying techniques provide, for each profile, information on the depth, the thickness and the gas saturation of the aquifer storage horizon, thereby allowing the localization of the gas edge. The accuracy of such surveys depends on the quality of the reflection of the seismic waves which is substantially determined by the material of the overburden.